1. Field of the Invention
This invention relates generally to implantable medical devices and, more particularly, to processing intracardiac electrocardiogram (EGM) signals acquired from implantable medical devices during a pacing event.
2. Description of the Related Art
Since the introduction of the first implantable pacemakers in the 1960s, there have been considerable advances in both the fields of electronics and medicine, such that there is presently a wide assortment of commercially available body-implantable electronic medical devices. The class of implantable medical devices now includes pacemakers, implantable cardioverters, defibrillators, neural stimulators, and drug administering devices, among others. Today's state-of-the-art implantable medical devices are vastly more sophisticated and complex than earlier ones. Today's state-of-the-art implantable medical devices are capable of performing significantly more complex tasks. The therapeutic benefits of such devices have been well proven.
As the functional sophistication and complexity of implantable medical device systems have increased over the years, it has become increasingly useful to include a system for facilitating communication between one implanted device and another implanted or external device, for example, a programming console, monitoring system, or the like. Shortly after the introduction of the earliest pacemakers, it became apparent that it would be desirable for physicians to non-invasively obtain information regarding the operational status of the implanted device, and/or to exercise at least some control over the device, e.g., to turn the device on or off or adjust the pacing rate, after implant. As new, more advanced features have been incorporated into implantable devices, it has been increasingly useful to convey correspondingly more information to/from the device relating to the selection and control of those features.
In particular, some of the important information relating to implantable pacemaker therapies concerns EGM signals. The EGM is the cardiac signal detected through the pacing electrodes. The amplification and filtering of the EGM signals for intrinsic events is well understood and is similar to sense amplifier signal processing. However, when pacing is performed as part of the implantable pacemaker therapies, the energies involved in the pacing are relatively high (on the order of volts) and the pulse widths are relatively short (on the order of msecs). The after-potential inherent in the lead system and the filter transient response mask the intrinsic events in conventional systems.
Paced EGM waveforms have historically had no value for discrimination and diagnosis of intrinsic events that could occur in a paced interval. The energy from pacing coupled through the bandpass and amplification detection circuitry results in signal amplitudes that conventionally prevent observation of cardiac events throughout the paced interval. Typically, the channel requires several intervals without pacing to settle down to a baseline that allows intrinsic cardiac event detection.
One conventional method to minimize pacing effects is to set the low frequency high-pass pole to a value of 18 Hz. However, one of the disadvantages of this method is that low frequency P-waves, R-waves and T-waves are distorted through differentiation. Similarly, filtering at greater fixed frequencies typically also provides unacceptable results.
Another conventional method is to reduce the gain of large signal amplitudes while maintaining sufficient gain for low amplitude signals, enabling observation of intrinsic cardiac events. This automatic gain control (AGC) function allows the channel to settle down toward the baseline more quickly than a fixed gain stage by limiting the large excursions of the output from the baseline. Moreover, by limiting the excursions of the amplifier circuitry from exceeding the common mode range, circuit recovery time is virtually eliminated. If the automatic gain control (AGC) function is mathematically described, then for any output, the input may be recreated mathematically.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.